The embodiment system of this invention generates ice slurry in direct contact heat transfer using two immiscible solvents one heavier than water and the other lighter than ice. The system comprises an ice generating tank producing ice by circulating immiscible light solvent as a cold heat transfer medium and a chiller that cools the light solvent stream. As for the ice generation tank of this invention, it generates ice slurry without ice adhesion problems. As for the chiller, it cools the light solvent stream without the problems of plugging by ice adhesion. The prior art is described below with an emphasis on the previous efforts having been made in the industry to resolve the problems of ice adhesion on cold surfaces that have been experienced in the ice slurry generators and solvent chillers.
Ice has been a favorite commodity in human life throughout the history. Innumerous attempts must have been tried to produce it in controllable ways. However, it still remains difficult to resolve the major problem of ice scale formation taking place on the cold heat transfer surfaces. Ice is made in the cold freezer boxes or in the ice makers having scrapers. The scrapers remove ice scale continuously, typically at 450 RPM, from the cold surfaces before the ice accumulates to a thickness at which it is hard to detach the scale due to the high adhesion forces along with a decrease of the heat transfer rates. Even with scrapers, the temperature driving force cannot go much larger than around 5° C., because with the larger temperature differences the ice scale formation becomes too severe to control. This difficulty limits the production capacity of ice makers. In addition, the ice makers with scrapers are mostly custom designed, and the units of larger capacities must be custom designed again rather than being expanded to larger sizes using the regular engineering scale-up rules. In order to circumvent these problems, new ideas of ice generation methods have been tried.
The new ideas have utilized the heat transfer techniques such as the evaporation of water as a refrigerant in vacuum, the direct contact heat transfer with refrigerants that vaporize in the water layer, the direct contact heat transfer with the immiscible solvents cooled by chillers, the super-cooling effect of water, and the fluidized bed freezers. With some exceptions, all these methods still have difficulties for commercial uses. The method of evaporation of water as a refrigerant in vacuum, for example, is in use most successfully for HVAC systems but only at temperatures near the freezing point of water for the applications in large capacities. The direct contact heat transfer with refrigerants or immiscible solvents experiences severe clogging problems due to the adhesion of ice on the cold surfaces around the distributor of cold medium along with many others. The method of using the super-cooling effect of water is not yet fully matured being unstable in operation with difficulties in control. The fluidized bed freezers are still in the development stage with problems similar to the scraped surface ice generators in ice adhesion.
The direct contact heat transfer using immiscible solvent still has a great possibility for ice slurry generation, once the ice clogging problems around the cold solvent distributor are resolved. When a cold solvent lighter than ice in density is injected into the water layer at the bottom of the tank, ice adheres on the cold surfaces of the solvent distributor quickly, resulting in clogging of nozzles. In order to resolve this problem, a solvent liquid heavier than water was injected into the water layer from the nozzles submerged in the heavy solvent layer at the bottom. The heavy solvent droplets shot into the water layer fell back quickly into the heavy solvent layer with a limited residence time resulting in insufficient heat transfer. When the heavy solvent is sprayed above the ice layer at the top of the tank in order to avoid ice clogging, it is difficult to have a controllable liquid distribution. When the solvent is sprayed below the ice layer, ice clogging occurs quickly again due to the adhesion of ice on the cold surfaces of the solvent distributor. The clogging problems around the distributor were well explained in the patent application (US 20050172659 A1), where a divergent inlet nozzle was proposed to resolve the problems.
In order to cool the circulating immiscible solvent stream, this invention uses a chiller having the solvent side heat transfer surfaces coated with a hydrophobic coating material. The hydrophobic coating prevents adhesion on the cold surfaces of the ice particles which are produced by freezing of the water molecules undissolving from the solvent stream. The prevention of ice adhesion is possible because the solvent functions as a lubricant on the cold surfaces allowing no area and residence time for the undis solving water droplets to sit on the cold surfaces and freeze to a sessile particle. The water molecules are produced by undissolving in the immiscible light solvent due to the decreasing solubility of water in the solvent while the solvent is cooled to lower temperatures. The undissolving effect is more significant for the immiscible solvents having high solubility of water than those having low solubility. For example, the solubility of water in toluene is in a range of around a hundred ppm, while that in perfluorohexane (C6F14) less than 10 ppm, and therefore the water freezing problems are more significant with toluene than perfluorohexane. The concerns about blockage of the passages in a chiller by freezing of the water entrained in the cold solvent were explained in the patent application (US 20050172659 A1), where a new type of inlet nozzle for the cold solvent feed was suggested to prevent such entrainment. The same problems must be expected from the water undissolving in solvent as those from the water entrained.
The effectiveness of the coated surfaces with the hydrophobic coating material was tested for icephobicity with a mixture of water and a freezing point depressant or an emulsion of water and oil, and found that the water froze with ice adhering on the cold surfaces. In order to improve the performance of the coated surfaces, a lubricant was applied on the surface and found that, in the atmosphere, the treated surfaces repelled water drops much better than the un-lubricated surfaces, but eventually lost the effectiveness while the lubricant was depleted due to the outside impacts such as those from a torrential rain. The loss of effectiveness was caused by penetration of air and water molecules into the pore structure of the surface as a result of the impacts, and then eventually the air molecules were replaced with water to wet the whole area. The liquid water molecules then became sessile on the surface and could have enough time for heat transfer to freeze. The lubricated hydrophobic surface was called SLIPS (slippery liquid-infused porous surfaces) that mimics the performance of a lotus leaf, and tested for the hydrophobic and icephobic effects mainly in the atmosphere.
Another factor to consider for prevention of ice adhesion in the chiller is the residence time for the water droplets of the undissolving molecules to sit on the cold surfaces for heat transfer to freeze. According to an experiment with a water drop of 2 micro-liters (μl), the drop is carried away by an air stream at the velocities above 5 m/s from the surface coated with superhydrophobic coating material. This means that a sufficient drag force can carry away the water droplets allowing no residence time to freeze on the cold surfaces. Actually, this phenomenon of particle removal makes the application in liquid phase more effective than in air, because the liquid flow can exert the equivalent drag forces at much lower velocities due to its high density compared to the air.
At the present time, no hydrophobic coating materials exist that exhibit icephobicity in actual applications at subzero conditions. In order for the hydrophobic surfaces to show icephobicity, a unique operational environment must be provided while in use in every particular application.
In summary, the ice slurry as a cold energy storage and transfer medium has been mostly used in applications requiring capacities lower than 100 KW (28 refrigeration tons) such as small food and fishing industries. This capacity is about the limit of the mechanical design for scraped surface ice slurry generators because of the ice adhesion problems. For wider applications in industry, however, higher capacities even up to 800 KW are usually necessary. However, the cost of multiple units hampers adoption of this option. The complexities in installation and maintenance are the other issues. A new generation method of simpler design at lower installation costs will stimulate the popularity of ice slurry as a cooling medium in the industries.